The present invention relates to a printed circuit board, and prepreg for a printed circuit board, and more particularly to a printed circuit board, and prepreg for a printed circuit board that are to be applied to usage which is used in the high-frequency band, and in which long-term reliability of electrical insulation, dielectric characteristics, and flexibility are requested, for example, a high-frequency antenna such as a low-dielectric loss three-dimensional antenna, a millimeter wave radar antenna, a high-frequency electronic component such as a high-frequency filter or a VCO, a base substrate for a high-frequency semiconductor device, and an IC package.
Recently, in accordance with the arrival of an advanced information society, communication means is remarkably changed, and information transmission tends to be made higher in speed and frequency. Also a mobile communication device such as a portable telephone, an automobile telephone, or a pager, and a new media system such as satellite broadcasting or satellite communication are in a stage of practical use, and expected to be used more widely in the future. In a new era of satellite broadcasting and satellite communication, particularly, it is requested to develop a printed circuit board for a high-frequency circuit such as a BS converter or an antenna which can cope with frequency bands from the microwave (1 to 30 GHz) region to the millimeter wave (30 to 300 GHz) region that is suitable for information transmission at a higher speed.
In the field of information processing, the clock frequency is made higher in order to attain high-speed calculation, and fusion with a computer is advanced by digitization of radio communication. In accordance with the above, use of a resin material which has dielectric characteristics of both small dielectric loss and low specific dielectric constant has been gradually studied. As an example, prepreg in which a fluororesin having excellent dielectric characteristics is used, and a printed circuit board made of such prepreg have been developed and received attention.
In a conventional printed circuit board using a fluororesin, as glass cloth serving as a basic material of prepreg, glass composition filaments of E-glass cloth (SiO2xe2x80x94Al2O3xe2x80x94CaO) are widely used because of excellent electrical insulation performance, high filament-forming properties, high workability, and a low price. However, such E-glass cloth has a specific dielectric constant of 6 to 7, and hence cannot sufficiently cope with low dielectric characteristics of a printed circuit board in which the speed-up of a calculation process is advancing. Moreover, E-glass cloth which is exclusively used in the conventional art has a mass of 50 g/m2 or larger. E-glass cloth having a mass of 50 g/m2 or larger is woven by a technique such as plain weave while using filament bundles in each of which about 200 glass filaments of several to about 10 microns are bundled, as warp and weft. As shown in a model diagram of FIG. 7, therefore, an increased number of glass filaments 21 inevitably increases the diameter D of a filament bundle 22 formed by bundling the filaments. Even when the filament bundle 22 of woven glass cloth is impregnated with an aqueous dispersion of a fluororesin, the resin cannot sufficiently penetrate into a center portion of the filament bundle 22, with the result that a resin-unimpregnated region 23 (the void region in FIG. 7 which is not hatched and which is surrounded by a circular broken line) is formed in the center portion. The resin-unimpregnated region 23 remains even after prepreg is produced, and even in the steps of processing a printed circuit board. In a chemical liquid immersing step which is one of the steps of processing a printed circuit board, therefore, the chemical liquid, metal ions, and the water content serving as an electrolyte easily impregnate or diffuse into a filament bundles passing through a through hole, via the resin-unimpregnated region 23. There is a possibility that this causes the electrical insulation resistance between through holes of the printed circuit board to be lowered and an insulation failure such as a short circuit occurs.
A printed circuit board for a high-frequency circuit is required to cope with a frequency band in the millimeter wave (30 to 300 GHz) region that is suitable to information transmission of a higher speed, and, on the other hand, also to have bendability and pliability (flexibility) which allows the printed circuit board to be used wound around a rod-like member serving as, for example, an antenna of a portable telephone or the like. In a printed circuit board of the conventional art using a fluororesin, however, E-glass cloth having a large mass and consisting of filament bundles in each of which a large number of or about 200 glass filaments are bundled is used as a basic material. Therefore, thinning and flexibility of a printed circuit board are impeded. A printed circuit board which has superior flexibility and is thin so as to be suitable for a high-frequency circuit has not yet been developed.
The present invention has been conducted in view of the above-mentioned background of the conventional art. It is an object of the present invention to provide a printed circuit board and prepreg for a printed circuit board which are excellent in dielectric characteristics, electrical insulation performance, and long-term reliability, which are thin and have superior flexibility, and which has many uses including a high-frequency circuit.
The printed circuit board of the present invention is a printed circuit board in which metal foil that forms a predetermined conductor pattern is placed on at least one face of a prepreg comprising: glass cloth serving as a basic material; and a fluororesin-impregnated layer in which a fluororesin is inpregnated and held into the glass cloth, and characterized in that the glass cloth has a mass of 30 g/m2 or less, and a thickness of a whole is set to 0.2 mm or less.
According to the thus configured printed circuit board of the present invention, glass cloth of 30 g/m2 or less is used as a basic material, and the thickness of the whole is set to 0.2 mm or less. Therefore, glass filaments constituting the glass cloth are small in number, and, as shown in a model diagram of FIG. 6, the diameter d and the section area of a filament bundle 22 formed by bundling glass filaments 21 can be reduced. According to this configuration, when the filament bundle 22 of woven glass cloth is impregnated with an aqueous dispersion of a fluororesin, the dispersion can sufficiently penetrate into a center portion of the filament bundle so that a resin-unimpregnated region is prevented from being formed in the center portion of the filament bundle. In a chemical liquid immersing step which is one of the steps of processing a printed circuit board, therefore, permeation and diffusion of the chemical liquid, metal ions, and the water content serving as an electrolyte, into the filament bundle can be blocked, and the electrical insulation resistance between through holes of the printed circuit board can be enhanced to attain remarkable improvements of electrical insulation performance and long-term reliability.
Since the mass of the glass cloth is reduced, moreover, the occupation ratio of the fluororesin of a lower specific dielectric constant in the prepreg of a constant size can be increased and the dielectric constant can be lowered by a degree corresponding to the increase. Therefore, it is possible to attain also an improvement of the dielectric characteristics.
Since the fluororesin can be uniformly impregnated into the glass cloth serving as the basic material, electrical insulation performance and dielectric characteristics can be prevented from being varied over the whole region of the prepreg and the printed circuit board, characteristics of the entire printed circuit board can be uniformalized, and the circuit board can be thinned.
In the glass cloth serving as the basic material in the printed circuit board of the present invention, preferably, glass composition filaments of E-glass cloth (SiO2xe2x80x94Al2O3xe2x80x94CaO) are used. More preferably, the glass cloth has a mass of 24 g/m2 or less. Furthermore, the glass cloth is preferably woven by filament bundles each of which is configured by 120 or less glass filaments.
Furthermore, in the printed circuit board of the present invention, a fluororesin-impregnated adhesive layer is preferably formed between the fluororesin-impregnated layer and the metal foil. The reasons for this will be described below.
When voids remain in the prepreg during steps of processing the printed circuit board, particularly the adhesiveness between the metal foil and the fluororesin-impregnated layer immediately below the metal foil is so poor that the peel strength is easily lowered. Atmospheric water, chemical liquid, or the like easily enters through the adhesive faces in which the peel strength is lowered. When the atmospheric water, chemical liquid, or the like permeates the remaining voids, there arises a problem in that a failure such as a short circuit of a circuit is secondarily caused. In a soldering step after such processing steps, gas or liquid which has entered voids may be suddenly expanded by the soldering temperature, thereby causing problems in that the portion is whitened to impair the appearance, and that breakage occurs in the circuit board.
When a fluororesin-impregnated adhesive layer is formed between the fluororesin-impregnated layer and the metal foil, the adhesiveness between the metal foil and the fluororesin-impregnated layer immediately below the metal foil is improved by the anchor effect due to the characteristics of the resin, and the peel strength can be enhanced. As a result, it is possible to obtain a printed circuit board in which the above-discussed secondary problem is solved and both the quality and the appearance are excellent.
As the fluororesin which is used in the printed circuit board of the invention, PTFE (tetrafluoroethylen resin), PFA (tetrafluoroethylene-phloroalkyl vinyl ether copolymer resin), FEP (tetrafluoroethylene-hexafluoropyran copolymer resin), or the like may be used. Among the resins, PTFE and PFA are preferably used from the viewpoints such as that the dielectric loss tangent is small, and that the melting point is so high that it does not melt during a soldering process. These fluororesins may be singly used, or combinedly used. In the case where a fluororesin-impregnated adhesive layer is formed between the fluororesin-impregnated layer and the metal foil, particularly, it is preferable to use PTFE as the fluororesin of the fluororesin-impregnated layer and PFA as that of the fluororesin-impregnated adhesive layer.
As the metal foil which is used in the printed circuit board of the invention, foil of a metal such as copper, aluminum, iron, stainless steel, nickel, or the like, or foil of an alloy of such metals may be used. Among the materials, copper foil is used most preferably.
The printed circuit board of the invention is not restricted to a circuit board in which a conductor pattern of metal foil is formed only on one face of the prepreg, and may be a circuit board in which a conductor pattern is formed on both the faces of the prepreg, or a circuit board of a multi-layer structure in which prepreg layers are stacked into, for example, four or eight layers on one face or both the faces, irrespective of the number of conductor patterns.
The prepreg for a printed circuit board of the invention is prepreg for a printed circuit board in which the prepreg comprises: glass cloth serving as a basic material; and a fluororesin-impregnated layer in which a fluororesin is impregnated and held into the glass cloth, and characterized in that the glass cloth has a mass of 30 g/m2 or less, and a diameter of one glass filament is set to 4 to 8 xcexcm.
According to the thus configured prepreg for a printed circuit board of the invention, glass cloth of 30 g/m2 or less may be used as a basic material, and the diameter of one glass filament may be set to 4 to 8 xcexcm, whereby the diameter and the section area of a filament bundle formed by bundling such glass filaments can be greatly reduced. As described above, an aqueous dispersion of a fluororesin sufficiently penetrates even into a center portion of the filament bundle. In a chemical liquid immersing step which is one of steps of processing a printed circuit board by using the prepreg, permeation and diffusion of the chemical liquid, metal ions, and the water content serving as an electrolyte, into the filament bundle are blocked. Prepreg in which remarkable improvements of electrical insulation performance and long-term reliability of a printed circuit board, and also an improvement of dielectric characteristics by increasing the occupation ratio of the fluororesin of a lower specific dielectric constant in prepreg of a constant size can be of course attained, and which are excellent in pliability and bendability is obtained. Thinning and flexibility of a printed circuit board which is produced by using such prepreg can be greatly improved. It is possible to produce a printed circuit board which can sufficiently cope with a use of being wound around a rod-like member that is used as, for example, an antenna of a portable telephone or the like, and which is applicable to any use such as a high-frequency circuit.
Also in the glass cloth serving as the basic material in the prepreg for a printed circuit board of the invention, preferably, glass composition filaments of E-glass cloth (SiO2xe2x80x94Al2O3xe2x80x94CaO) are used. More preferably, the glass cloth has a mass of 24 g/m2 or less. Furthermore, the glass cloth is preferably woven by filament bundles each of which is configured by 120 or less glass filaments.